Recently, electronic products have been improved to be lighter in weight, smaller in size, and higher in density, thereby resulting in an increase in the demand for various types of substrates for printed wringing boards, especially, the demand for flexible laminate (which may be referred to as flexible printed wiring boards (hereinafter FPCs) or the like later). The flexible laminate has such a structure that a circuit is formed with a metal foil on an insulating film.
A flexible laminate is generally produced by adhering a metal foil on a substrate by heat and pressure applications with an adhesive material of various kinds therebetween. The substrate is an insulating film being flexible and made of an insulating material of various kinds. The insulating film is preferably a polyimide film or the like. The adhesive material is generally a heat-curable adhesive agent, which is epoxy-based, acryl-based, or the like. (The FPC in which such a heat-curable adhesive agent is used may be refereed to as a three-layered FPC).
The heat-curable adhesive agent is advantageous in that it allows adhesion at a relatively low temperature. However, it is expected that the three-layered FPCs cannot sufficiently satisfy future demands for better heat resistance, flexibility, and electric reliability.
In view of this, FPCs in which a metal layer is formed directly on the insulting film, or in which the adhesive layer is made of a thermoplastic polyimide have been proposed (hereinafter, these FPCs may be referred to as two-layered FPCs). The two-layered FPCs show better characteristics than the three-layered, and higher demands for the two-layered FPCs are expected.
A flexible metal-clad board for a two-layered FPC may be prepared by a casting method, metalizing method, or a laminating method, for example. In the casting method, a polyamide acid, which is a precursor of a polyimide, is cast and spread on a metal foil and is imidated. In the metalizing method, a metal layer is formed directly on a polyimide film by sputtering or plating. In the laminating method, a polyimide film and a metal foil are adhered together via a thermoplastic polyimide.
The laminating method is more excellent than the others because it can provide a wider range of thickness of the metal foil than does the casting method and requires a lower apparatus cost than the metalizing method. As an apparatus for the lamination, heat roll laminating apparatus, double belt press apparatus or the like is used, in which materials in a rolled form are continuously unrolled to be fed in for the lamination. For better productivity, the heat roll lamination method is more preferable among them.
In the production of the conventional three-layered FPC by lamination method, it is possible to carry out the lamination at lamination temperatures less than 200° C. (see Patent citation 1), because a thermosetting resin is used to form the adhesive layer. On the other hand, a thermoplastic resin is used to form the adhesive layer in the two-layered FPC. Therefore, it is necessary for the two-layered FPC to apply a high temperature of 200° C. or higher, in some cases, a temperature of approximately 400° C., in order to heat the adhesive layer to be adhesive.
Incidentally, heat roll lamination method is carried out by conveying a metal foil and an adhesive film through a pair of rollers thereby to press the metal film and the adhesive film together. Therefore, stress is applied on the adhesive film to be extended in its longitudinal direction (hereinafter, “MD direction”), but to be compressed in its width direction (hereinafter, “TD direction”). This stress induces distortion remained in the resultant laminate film. Such distortion would result in dimensional changes when the laminate film is wired by etching and when solder reflow is carried out for mounting elements on the laminate film. More specifically, the heat roll lamination applies a force on the film to swell it in the MD direction but to shrink it in the TD direction. As a result, this distortion is released when etching the metal foil away from a flexible board to be wired and when heating the flexible printed wiring board by solder reflow. The release of the distortion causes shrinkage in the MD direction and swelling in the TD direction.
Recently, the wiring on the substrates is getting finer and finer for the sake of miniaturization and light weight of electronic devices. Accordingly, smaller elements are mounted in higher densities. Therefore, a large dimensional change after the formation of the fine wiring would displace the wires from positions to which the elements are to be mounted according to design, thereby failing to connect the elements with the substrate sufficiently.
In view of this, the following arts for suppressing the dimensional changes have been proposed: (1) arts proposing preferable conditions for the lamination process; and (2) arts proposing preferable ranges of thermal shrinkage ratio of the polyimide film.
The arts proposing preferable conditions for the lamination process are exemplified by an art proposing a pressure to be applied in laminating with pressure and heat application (as described in Patent citation 3), an art proposing a force of tension applied on an adhesive film (as described in Patent citation 2), the other arts. Moreover, the arts proposing preferable ranges of thermal shrinkage ratio of the polyimide film are exemplified by an art proposing an upper limit of the thermal shrinkage ratio of a tape-shape film made of an aromatic polyimide (as described in Patent citation 4), an art proposing an upper limit of the thermal shrinkage ratio of a polyimide film at 300° C., which becomes adhesive by heat and pressure application, an art proposing a lower limit of young modulus and upper limits of the thermal shrinkage ratio at 300° C. and 450° C., and the other arts.
[Patent Citation 1]
Japanese Unexamined Patent Application Publication, Tokukaihei, No. 9-199830 (published on Jul. 31, 1997).
[Patent Citation 2]
Japanese Unexamined Patent Application Publication, Tokukai, No. 2002-326280 (published on Nov. 12, 2002).
[Patent Citation 3]
Japanese Unexamined Patent Application Publication, Tokukai, No. 2002-326308 (published on Nov. 12, 2002).
[Patent Citation 4]
Japanese Unexamined Patent Application Publication, Tokukaihei, No. 10-77353 (published on Mar. 24, 1998).
[Patent Citation 5]
Japanese Unexamined Patent Application Publication, Tokukai, No. 2001-270034 (published on Oct. 2, 2001).
[Patent Citation 6]
Japanese Unexamined Patent Application Publication, Tokukai, No. 2003-335874 (published on Nov. 28, 2003).
However, all the conventional arts do not take into consideration that the flexible printed wiring board prepared via the heat roll lamination process shrinks in the MD direction but expands in the TD direction in etching or heating. Therefore, the conventional arts are not yet practical.
More specifically, for example, the arts disclosed in Patent citations 2 and 3 can sufficiently solve the problems of the dimensional change in the adhesive film in some usages. However, in some cases, the arts disclosed in Patent citations 2 and 3 cannot provide much finer miniaturization and much higher density of the electronic elements that are required recently.
On the other hand, the art disclosed in Patent citation 4 requires that the aromatic polyimide film and the copper foil are adhered together by the adhesive agent, meanwhile the art disclosed in Patent citation 5 requires that the aromatic polyimide layer that becomes adhesive when heat and pressure are applied thereon is provided on at least one surface or preferably both surfaces of a highly heat resistant aromatic polyimide layer. In other words, these arts are suitable for the three-layered FPC but not for the two-layered FPC. Moreover, Patent citation 6 does not mention as to whether the art disclosed therein is suitable for the two-layered FPC or the three-layered FPC, and does not discuss the problem associated with the heat roll lamination.
Therefore, the arts disclosed in Patent citations 4 to 6 do not take the shrinkage in the MD direction and expansion in the TD direction into consideration as the arts disclosed in Patent citations 1 to 2. Thus, in some cases, the arts disclosed in Patent citations 4 to 6 cannot provide finer miniaturization and higher density of the electronic elements that are required recently.